It is very important that you harden off any greenhouse seedlings before planting them permanently outside. This is also true for any purchased nursery plants that have been sheltered from direct sunlight. It’s kind of like a tan versus a sunburn, only it is much more life threatening for plants. (click on any photo to enlarge)
Even though plants can grow well under greenhouse lights, the strength of those lights is not anywhere near the intensity of sunlight. Thus, in the greenhouse, their leaves grow to take in as much of this light as possible. This means that leaves that grow in the greenhouse, or under shelter, will be shocked or burnt when all of a sudden subjected to long periods of direct sunlight. Without functional leaves, a plant will die.
If you grow the exact same plant in the greenhouse and outdoors, you can see the difference in the leaves. Usually the outdoor plants will seem smaller, stockier, and darker. The greenhouse plant leaves will be broader, lighter green, and even feel more tender. (Click here to read more about starting plants from seed.)
Sunlight is not the only environmental factor that the plants need to get used to with hardening off. They also need to toughen up to wind. It’s kind of like developing their abdominal muscles from trying to keep their balance. In the greenhouse, there may be fans for air movement, but there is nothing like a stiff breeze or the nearly continual winds that can go outdoors.
To prepare a greenhouse seedling or plant for life in the wild outdoors, it must be gradually hardened off. This can take anywhere from a couple of weeks to a couple of months, depending on the weather and how consistent you can be in the process.
For the first days of hardening off, I try to watch for very mild, sunny weather. I don’t want the plants to be subject to too much of a temperature difference, but I don’t want it scorching hot, either. It’s is generally about 75°F my greenhouse, so about 60-65°F outside is what I aim for outside.
A little bit of overcast can be useful, as long as it isn’t too cold and blustery. The best first hardening off time is most often going to be an afternoon event for hardening off anything in March or early April here in southwest Idaho, which is when I begin for some things. In late April and or early May, I tend to utilize morning hours more.
I use a kitchen timer faithfully if my plants are only ready to spend a few hours outside. Their lives depend on it. Then, I don’t turn it off until I am actively attending to relocating my seedlings. If I need a few more minutes to get to it, I set the timer again. It is too easy to lose track of time while doing something else.
I usually harden my seedlings off in increments of hours. That is, I will put them out for one hour the first day, then maybe two hours the second day. But I don’t necessarily always increase the amount of time. I will often put them outside for the same length of time for 2-3 days in a row. I consider them still tender until about 6 hours is reached.
It is not necessary that the plants go outside every day for the hardening off to be effective. If there is a day or two where the weather or your schedule are not cooperative, simply start at the same amount of time or slightly less than the last time you took them out. It all adds up to strengthen the plants.
Another variation to hardening off seedlings is to put them outside once they begin to sprout. Sprouting in pots is usually more productive if they are kept at constant temperatures and moisture. This is easiest to maintain indoors in the earlier parts of spring for some plants. However, they can handle the sunlight from the get-go if you get them out there right as they show their pretty little green faces. Keep in mind, though, that they are still very vulnerable to drying out. Think of watering them like gentle spring rains, especially until they have more root and leaf growth. Also, the outdoor temperatures need to be consistently within a range that the plants will grow in. (Click here for ideas about deciding which plants to start in pots)
Sometimes I will put more tender plants in a place that will get some dappled sun or shade sooner than the others, particularly if it is a bright, sunny, warm day. If you study the patterns of sunlight during the day in your preferred hardening off zone, you will likely discover micro-environments that can work to your plants’ advantage.
As the time spent outside gets longer, closer attention needs to be paid to water needs. Not only does the direct sunlight lead to more evaporation, but the plants usually have more foliage, which uses up the water faster. Since they are still in relatively small pots, they need to be monitored.
One thing that has to be guarded against is unexpected downpours. The force of such water can wreak havoc on small seedlings in general, but it also can displace soil from the pots or threaten to drown them because the pots cannot drain quickly enough. If there is any chance of such a rain while I am gone from the house or otherwise unable to keep a close eye on my plants, I put as many plants as possible under the eaves of the house for protection, but where they still get sunlight. I have learned the hard way, though, to make sure they are pushed far enough under so that gutter overflow doesn’t cascade down onto my plants.
If my plants do get subjected to a hard rain, it is important to dump any extra water out that has collected in their trays, if they are in such groupings. Even if the pots can theoretically drain, they won’t if the tray is full of water. A tray can collect a lot of water in a good rain, which can take too long to evaporate for the health of the plant.
Once the plants are up to 6 hours outside during the day, they can handle some warm nights outside, too. I use 40°F as my benchmark nighttime low for this, but also evaluate wind chill. Keep in mind that because my patio is south facing, it probably actually stays warmer than that. Plus, being close to the house and somewhat protected from the sides, there is a pocket of warmth there.
When the nights are warm enough, I can leave the plants out 24/7, but I watch the nighttime temperatures diligently. I haven’t spent all that effort growing those plants just to lose them in one cold night! I also watch for other weather factors and place the plants in more protected places as seems wise, i.e. under the eaves or benches, to be able to sleep more peacefully. I now have some plant shelves my husband made me specifically for this purpose, which helps me out a lot.
You can see there is no secret science to the process. It is just a matter of diligence and monitoring. Taking time to properly harden off your plants will not take very much effort compared to the whole of the gardening season and will be well worth doing right. Soon you will have plants that are ready to live the rest of the season outdoors, where you can spend as much time as possible, too.